System for precharging a DC link in a variable speed drive

ABSTRACT

A variable speed drive with a converter that is controllable to precharge a DC link is provided. The variable speed drive also includes an inverter. The converter converts a fixed line frequency, fixed line voltage AC power from an AC power source into DC power. The DC link filters the DC power from the converter. Finally, the inverter is connected in parallel with the DC link and converts the DC power from the DC link into a variable frequency, variable voltage AC power. The converter includes a plurality of pairs of power switches, wherein each pair of power switches includes a reverse blocking power switch connected in anti-parallel to another reverse blocking power switch.

BACKGROUND OF THE INVENTION

The present invention relates generally to variable speed drives. Morespecifically, the present invention relates to a system for prechargingthe DC link in a variable speed drive using insulated gate bipolartransistors in the rectifier or converter.

A variable speed drive (VSD) for heating, ventilation, air-conditioningand refrigeration (HVAC&R) applications typically includes a rectifieror converter, a DC link, and an inverter. The rectifier or converterconverts the fixed line frequency, fixed line voltage AC power from anAC power source into DC power. The DC link filters the DC power from theconverter and typically contains a large amount of electricalcapacitance. Finally, the inverter is connected in parallel with the DClink and converts the DC power from the DC link into a variablefrequency, variable voltage AC power. When electric power is applied tothe VSD, the voltage across the DC link capacitors, referred to as theDC link voltage, rises from zero to a rated value, typically around 600V. If this rise of the DC link voltage were left to occur naturally, itwould happen very quickly by drawing very large electric currents fromthe input power lines, through the rectifier, and into the DC linkcapacitors. This large current, referred to as an inrush current, can bedamaging to the components of the VSD. Thus, to avoid damage to the VSDcomponents, the rise of the DC link voltage from 0 V to the ratedvoltage has to be accomplished in some controlled manner. Thiscontrolled raising of the DC link voltage is referred to as a DC linkprecharge operation.

Most VSDs accomplish a DC link precharge by two different methods. Thefirst method employs precharge resistors and contactors connectedbetween the input power line and the rectifier. The second methodemploys a rectifier consisting (at least partially) of thyristors, alsocalled silicon controlled rectifiers, or SCRs.

In the first method, a precharge contactor is used to connect prechargeresistors between the input power line and the rectifier or, sometimes,between the input power line and the DC link. These precharge resistorslimit the inrush current to a manageable level. After the precharge iscompleted, the precharge resistors are excluded from the circuit byopening the precharge contactor, and the input power line is connecteddirectly to the rectifier by closing another contactor, referred to asthe supply contactor. The supply contactor remains closed during theoperation of the system. This method is well suited for VSDs in whichthe rectifier is a simple diode rectifier, which offers no means forcontrolling the inrush current. The main disadvantage of this method isin the cost and size of its components, in particular of the supplycontactor, which can negatively impact the cost and size of the entireVSD.

In the second method, the rectifier itself is used to accomplishprecharge. The rectifier in this case has at least one SCR in eachphase. SCRs are power semiconductors whose current conduction can beelectronically controlled. The conduction of the rectifier's SCRs iscontrolled so as to let only small pulses of inrush current flow duringprecharge. After the precharge is completed, the rectifier's SCRs arecontrolled to conduct at all times, i.e., the rectifier after theprecharge acts as if it were a diode rectifier.

The two precharge methods described above are applicable to VSDs whoserectifiers are made up of diodes and/or SCRs. However, there are VSDswith rectifiers or converters that do not use diodes or SCRs, but infact, use insulated gate bipolar transistors (IGBTs) or other types ofpower switches or transistors. The IGBTs are usually packaged in modulesand it is common for one module to include six IGBTs, which would beadequate for a three-phase rectifier. It is noted that the IGBT modulecan also be used for the inverter of the VSD. The typical IGBT moduleincludes a diode for every IGBT present in the IGBT module, i.e., therewould be six diodes in an IGBT module with six IGBTs. These diodes arecommonly referred to, and connected, as anti-parallel diodes and areused to conduct current after an IGBT is turned off when the VSDoperates in pulse width modulating (PWM) mode. The six anti-paralleldiodes in the IGBT module can be considered to form a three-phase dioderectifier that is embedded within the IGBT module.

The embedded diode rectifier presents a problem for the precharge ofVSDs that use IGBT modules because the first precharge method (prechargeand supply contactors and resistors) described above must be used toprecharge the DC link. This places additional cost and size burden onVSDs having IGBT modules for the rectifier or converter.

Therefore, what is needed is a system for precharging the DC link of aVSD having an IGBT-based rectifier or converter that does not requireprecharge and supply contactors and resistors.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a variable speeddrive including an inverter module, a DC link and a converter module.The inverter module can convert a DC voltage to an AC voltage to power amotor. The DC link can filter and store energy and is electricallyconnected in parallel to the inverter module. The converter module canconvert an AC voltage to a DC voltage and is electrically connected inparallel to the DC link and is electrically connected to an AC powersource. The converter module includes a plurality of pairs of powerswitches, wherein each pair of power switches includes an insulated gatebipolar transistor connected to an anti-parallel diode and a reverseblocking insulated gate bipolar transistor connected to an anti-parallelreverse blocking insulated gate bipolar transistor. The plurality ofpairs of power switches in the converter module are controllable toprecharge the DC link.

One advantage of the present invention is that it is small and compactand can thereby reduce the size of the variable speed drive.

Another advantage of the present invention is that it reduces the costof the variable speed drive by eliminating the need for expensive parts.

Still another advantage of the present invention is that it increasesthe reliability of the variable speed drive by eliminatingelectromechanical parts subject to routine wear and tear.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate schematically general system configurationsof the present invention.

FIGS. 2A and 2B illustrate schematically embodiments of variable speeddrives of the present invention.

FIG. 3 illustrates schematically a refrigeration system that can be usedwith the present invention.

FIGS. 4A and 4B illustrate a circuit diagram of an embodiment of thevariable speed drive of the present invention.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B illustrate generally system configurations of thepresent invention. An AC power source 102 supplies a variable speeddrive (VSD) 104, which powers a motor 106 (see FIG. 1A) or motors 106(see FIG. 1B). The motor(s) 106 is preferably used to drive acorresponding compressor of a refrigeration or chiller system (seegenerally, FIG. 3). The AC power source 102 provides single phase ormulti-phase (e.g., three phase), fixed voltage, and fixed frequency ACpower to the VSD 104 from an AC power grid or distribution system thatis present at a site. The AC power source 102 preferably can supply anAC voltage or line voltage of 200 V, 230 V, 380 V, 460 V, or 600 V, at aline frequency of 50 Hz or 60 Hz, to the VSD 104 depending on thecorresponding AC power grid.

The VSD 104 receives AC power having a particular fixed line voltage andfixed line frequency from the AC power source 102 and provides AC powerto the motor(s) 106 at a desired voltage and desired frequency, both ofwhich can be varied to satisfy particular requirements. Preferably, theVSD 104 can provide AC power to the motor(s) 106 having higher voltagesand frequencies and lower voltages and frequencies than the ratedvoltage and frequency of the motor(s) 106. In another embodiment, theVSD 104 may again provide higher and lower frequencies but only the sameor lower voltages than the rated voltage and frequency of the motor(s)106. The motor(s) 106 is preferably an induction motor, but can includeany type of motor that is capable of being operated at variable speeds.The induction motor can have any suitable pole arrangement including twopoles, four poles or six poles.

FIGS. 2A and 2B illustrate different embodiments of the VSD 104 of thepresent invention. The VSD 104 can have three stages: a converter stage202, a DC link stage 204 and an output stage having one inverter 206(see FIG. 2A) or a plurality of inverters 206 (see FIG. 2B). Theconverter 202 converts the fixed line frequency, fixed line voltage ACpower from the AC power source 102 into DC power. The DC link 204filters the DC power from the converter 202 and provides energy storagecomponents. The DC link 204 can be composed of capacitors and inductors,which are passive devices that exhibit high reliability rates and verylow failure rates. Finally, in the embodiment of FIG. 2A, the inverter206 converts the DC power from the DC link 204 into variable frequency,variable voltage AC power for the motor 106 and, in the embodiment ofFIG. 2B, the inverters 206 are connected in parallel on the DC link 204and each inverter 206 converts the DC power from the DC link 204 into avariable frequency, variable voltage AC power for a corresponding motor106. The inverter(s) 206 can be a power module that can include powertransistors, insulated gate bipolar transistor (IGBT) power switches andinverse diodes interconnected with wire bond technology. Furthermore, itis to be understood that the DC link 204 and the inverter(s) 206 of theVSD 104 can incorporate different components from those discussed aboveso long as the DC link 204 and inverter(s) 206 of the VSD 104 canprovide the motors 106 with appropriate output voltages and frequencies.

With regard to FIGS. 1B and 2B, the inverters 206 are jointly controlledby a control system such that each inverter 206 provides AC power at thesame desired voltage and frequency to corresponding motors based on acommon control signal or control instruction provided to the inverters206. In another embodiment, the inverters 206 are individuallycontrolled by a control system to permit each inverter 206 to provide ACpower at different desired voltages and frequencies to correspondingmotors 106 based on separate control signals or control instructionsprovided to each inverter 206. This capability permits the inverters 206of the VSD 104 to more effectively satisfy motor 106 and system demandsand loads independent of the requirements of other motors 106 andsystems connected to other inverters 206. For example, one inverter 206can be providing full power to a motor 106, while another inverter 206is providing half power to another motor 106. The control of theinverters 206 in either embodiment can be by a control panel or othersuitable control device.

For each motor 106 to be powered by the VSD 104, there is acorresponding inverter 206 in the output stage of the VSD 104. Thenumber of motors 106 that can be powered by the VSD 104 is dependentupon the number of inverters 206 that are incorporated into the VSD 104.In one embodiment, there can be either 2 or 3 inverters 206 incorporatedin the VSD 104 that are connected in parallel to the DC link 204 andused for powering a corresponding motor 106. While the VSD 104 can havebetween 2 and 3 inverters 206, it is to be understood that more than 3inverters 206 can be used so long as the DC link 204 can provide andmaintain the appropriate DC voltage to each of the inverters 206.

FIG. 3 illustrates generally one embodiment of the present inventionincorporated in a refrigeration or chiller system using the systemconfiguration and VSD 104 of FIGS. 1A and 2A. As shown in FIG. 3, theHVAC, refrigeration or liquid chiller system 300 includes a compressor302, a condenser arrangement 304, a liquid chiller or evaporatorarrangement 306 and the control panel 308. The compressor 302 is drivenby motor 106 that is powered by VSD 104. The VSD 104 receives AC powerhaving a particular fixed line voltage and fixed line frequency from ACpower source 102 and provides AC power to the motor 106 at desiredvoltages and desired frequencies, both of which can be varied to satisfyparticular requirements. The control panel 308 can include a variety ofdifferent components such as an analog to digital (A/D) converter, amicroprocessor, a non-volatile memory, and an interface board, tocontrol operation of the refrigeration system 300. The control panel 308can also be used to control the operation of the VSD 104, and the motor106.

Compressor 302 compresses a refrigerant vapor and delivers the vapor tothe condenser 304 through a discharge line. The compressor 302 can beany suitable type of compressor, e.g., screw compressor, centrifugalcompressor, reciprocating compressor, scroll compressor, etc. Therefrigerant vapor delivered by the compressor 302 to the condenser 304enters into a heat exchange relationship with a fluid, e.g., air orwater, and undergoes a phase change to a refrigerant liquid as a resultof the heat exchange relationship with the fluid. The condensed liquidrefrigerant from condenser 304 flows through an expansion device (notshown) to the evaporator 306.

The evaporator 306 can include connections for a supply line and areturn line of a cooling load. A secondary liquid, e.g., water,ethylene, calcium chloride brine or sodium chloride brine, travels intothe evaporator 306 via return line and exits the evaporator 306 viasupply line. The liquid refrigerant in the evaporator 306 enters into aheat exchange relationship with the secondary liquid to lower thetemperature of the secondary liquid. The refrigerant liquid in theevaporator 306 undergoes a phase change to a refrigerant vapor as aresult of the heat exchange relationship with the secondary liquid. Thevapor refrigerant in the evaporator 306 exits the evaporator 306 andreturns to the compressor 302 by a suction line to complete the cycle.It is to be understood that any suitable configuration of condenser 304and evaporator 306 can be used in the system 300, provided that theappropriate phase change of the refrigerant in the condenser 304 andevaporator 306 is obtained.

The HVAC, refrigeration or liquid chiller system 300 can include manyother features that are not shown in FIG. 3. These features have beenpurposely omitted to simplify the drawing for ease of illustration.Furthermore, while FIG. 3 illustrates the HVAC, refrigeration or liquidchiller system 300 as having one compressor connected in a singlerefrigerant circuit, it is to be understood that the system 300 can havemultiple compressors, powered by a single VSD as shown in FIGS. 1B and2B or multiple VSDs, see generally, embodiment shown in FIGS. 1A and 2A,connected into each of one or more refrigerant circuits.

FIGS. 4A and 4B show a circuit diagram for one embodiment of the VSD104, as shown in FIG. 2A. In this embodiment of the VSD 104, the inputlines L1-L3 from the three-phase AC power source 102 are connected to acircuit breaker 402, which circuit breaker 402 can disconnect the VSD104 from the AC power source 102 when an excess current, voltage orpower is provided to the VSD 104. The circuit breaker 402 can then beconnected to an optional autotransformer 404. The autotransformer 404,when used, is preferably used to adjust an input voltage (either up ordown) from the AC power source 102 to a desired input voltage. Fuses 406for each line can be used to disconnect that input phase or line of theVSD 104 in response to an excessive current in that line. Inductors 408for each line are used to smooth the current in the corresponding lineof the VSD 104. The output of each of the inductors 408 is then providedto the converter 202 to convert each phase of the input AC power to DCpower.

Connected in parallel to the outputs of the converter 202 is the DC link204. The DC link 204 in this embodiment includes capacitors 420 andresistors 422 to filter the DC power and store energy from the DC bus412. The resistors 422 can function as voltage balancing devices tomaintain a substantially equal DC link voltage between capacitor banks420. The resistors 422 can also function as charge depleting devices to“bleed off” stored voltage in the capacitor banks 420 when the power isremoved from the AC power source 102. Also connected to the DC bus 412is an inverter section 206, which converts the DC power on the DC bus412 to three phase AC power for a motor. In the embodiment shown inFIGS. 4A and 4B, one inverter section or module 206 is used. However,additional inverter modules 206, as shown in FIG. 2B, can be added andwould have a similar circuit representation to the inverter module 206shown in FIG. 4B. The inverter module 206 includes three pairs (one foreach output phase) of IGBT power switches and inverse diodes. Theinverter modules 206 also include the corresponding control connectionsto control the switching of the IGBT power switches.

The inverter module 206 converts the DC power on the DC bus 412 to threephase AC power by selectively switching each of the IGBT power switchesin the inverter module 206 between an “on” or activated position and an“off” or deactivated position using a modulation scheme to obtain thedesired AC voltage and frequency from the inverter module 206. A gatingsignal or switching signal is provided to the IGBT power switches by thecontrol panel 308, based on the modulation scheme, to switch the IGBTpower switches between the “on” position and the “off” position. TheIGBT power switches are preferably in the “on” position when theswitching signal is “High,” i.e., a logical one, and in the “off”position when the switching signal is “Low,” i.e., a logical zero.However, it is to be understood that the activation and deactivation ofthe IGBT power switches can be based on the opposite state of theswitching signal.

In a preferred embodiment of the present invention, the precharge of thecapacitors 420 of the DC link 204 is controlled using the convertermodule 202 shown in FIG. 4A. The converter module 202 includes threepairs (one pair for each input phase) of power switches or transistors.The converter module 202 also includes the corresponding controlconnections (not shown for simplicity) to control the switching of thepower switches in a manner similar to that described above for theinverter module 206. In a preferred embodiment of the converter module202, the power switches are IGBT power switches, as discussed in detailbelow, that are controlled by a pulse width modulation technique togenerate the desired output voltages for the DC link. Preferably, theconverter module 202 can operate as a boost rectifier to provide aboosted DC voltage to the DC link 204 to obtain an output voltage fromthe VSD 104 greater than the input voltage of the VSD 104.

In the converter module 202, one of the power switches in each pair ofpower switches is an IGBT 450 connected to an inverse or anti-paralleldiode 452. The inverse or anti-parallel diode 452 is used to conductcurrent after the other power switch, IGBT 454, is turned off when theVSD 104 is operated in a pulse width modulation mode. As shown in FIG.4A, the IGBTs 450 and inverse diodes 452 are connected between theoutput of the inductors 408 and the negative rail of the DC bus 412.However, in another embodiment of the present invention, the IGBTs 450and inverse diodes 452 can be connected between the output of theinductors 408 and the positive rail of the DC bus 412.

The other power switch in the pair of power switches is a reverseblocking IGBT 454, i.e., the IGBT 454 is capable of blocking voltages inthe reverse as well as the forward direction. The reverse blocking IGBT454 is connected to an inverse or anti-parallel IGBT 456, whichanti-parallel IGBT 456 is also a reverse blocking IGBT. Theanti-parallel IGBT 456 is then preferably controlled during theprecharge operation to permit only small pulses of inrush current toreach the DC link. After the precharge operation is completed, theanti-parallel IGBT 456 can be controlled to conduct at all times,similar to the anti-parallel diode 452. In another embodiment of thepresent invention, other reverse blocking power switches, such as anIGBT power switch, e.g., IGBT 450, connected in series with a diode thatcan provide reverse blocking, can be used instead of reverse blockingIGBTs 454. In still another embodiment of the present invention, IGBTs450 can be replaced by reverse blocking IGBTs 454.

The reverse blocking IGBT 454 blocks a positive emitter-to-collectorvoltage that is approximately equal to the peak line-to-line voltagethat appears across the IGBT 454 for as long as the conduction of theanti-parallel IGBT 456 is delayed for the purpose of precharge. Inaddition, the reverse blocking capabilities of the reverse blocking IGBT454 and the anti-parallel IGBT 456 provide good reverse recoverycharacteristics when operated as conventional diodes. The reverserecovery characteristics of the anti-parallel IGBT 456 preventsignificant reverse recovery losses from occurring in the anti-parallelIGBT 456 by preventing a significant reverse current from flowing in theanti-parallel IGBT 456 whenever the series connected IGBT 450 in thesame phase turns on. Furthermore, the preventing of the reverse currentin the anti-parallel IGBT 456 can limit the peak current value, and thecorresponding losses, in the series connected IGBT 450 when seriesconnected IGBT 450 is turned on. As shown in FIG. 4A, the reverseblocking IGBT 454 and anti-parallel IGBT 456 are connected between theoutput of the inductors 408 and the positive rail of the DC bus 412.However, in another embodiment of the present invention, the reverseblocking IGBT 454 and anti-parallel IGBT 456 can be connected betweenthe output of the inductors 408 and the negative rail of the DC bus 412.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A variable speed drive comprising: an inverter module to convert a DCvoltage to an AC voltage to power a motor; a DC link to filter and storeenergy, the DC link being electrically connected in parallel to theinverter module; a converter module to convert an AC voltage to a DCvoltage, the converter module being electrically connected in parallelto the DC link and being electrically connected to an AC power source,the converter module comprising a plurality of pairs of power switches,wherein each pair of power switches includes an insulated gate bipolartransistor connected to an anti-parallel diode and a reverse blockinginsulated gate bipolar transistor connected to an anti-parallel reverseblocking insulated gate bipolar transistor; and wherein the plurality ofpairs of power switches in the converter module are controllable toprecharge the DC link.